1. Put oven rack in middle position and preheat oven to 350 degrees. Grease a springform pan with some oil. Then, line bottom with a round of parchment paper. Oil parchment.
2. Finely grate enough lemon zest to measure 1-1/2 teaspoons and whisk together with flour. Halve lemon; then squeeze and reserve 1-1/2 tablespoons fresh lemon juice.
3. Beat together yolks and 1/2 cup sugar in a large bowl with an electric mixer at high speed until thick and pale, about 3 minutes. Reduce speed to medium and add olive oil and reserved lemon juice, beating until just combined (mixture may appear separated). Using a wooden spoon, stir in flour mixture (do not beat) until just combined.
4. In another large bowl and using clean beaters, beat egg whites (from 4 eggs) with salt at a medium-high speed until foamy. Then, add 1/4 cup sugar a little at a time, beating, and continue to beat until egg whites just hold soft peaks, about 3 minutes.
5. Gently fold 1/3 of whites into yolk mixture to lighten. Then, fold in remaining whites gently, but thoroughly.
6. Transfer batter to the springform pan and gently rap against work surface once or twice to release any air bubbles. Sprinkle top evenly with remaining 1-1/2 tablespoons sugar. Bake until puffed and golden, about 45 minutes or when a toothpick inserted in center of cake comes out clean.
7. Cool cake in pan on a rack for 10 minutes. Then, run a thin knife around edge of pan and remove side of pan. Cool cake to room temperature, about 1-1/4 hours. Remove bottom of pan and peel off parchment.
8. For the reduction: In a small saucepan, add vanilla-fig balsamic and reduce by half, or until syrupy. Set aside, but don't allow to cool completely.
9. Transfer cake to a serving plate. Drizzle individual slices with vanilla-fig balsamic reduction and enjoy!